This invention relates to a method of improving the rheological properties of clays, more especially kaolin clays, in water, to a pigment composition for use in aqueous dispersions, for example in paper coating compositions, and to a method of preparing an aqueous suspension containing from about 60% to about 75% by weight of a clay mineral and having improved rheological properties.
Many varieties of coated papers are produced today. They are principally coated with a composition, sometimes known as the coating colour, which essentially comprises an adhesive, also known as a binder, and a pigment. A discussion of the constituents of paper coating compositions and of the methods of applying such compositions to paper is given in Chapter XIX, Volume III, of the 2nd Edition of the book by James P. Casey entitled "Pulp and Paper: Chemistry and Technology". The adhesive used can be, for example, starch, casein or a synthetic resin latex; the particular adhesive used will depend, for example, on the printing process to be used, e.g. offset lithography requires the adhesive to be water-insoluble. Generally, the pigment will consist of clay together with an amount, which may be up to 60% by weight, of one or more other constituents, for example, calcium carbonate, calcium sulphate, lithopone, barium sulphate, titanium pigments, talc or satin white.
In certain procedures for preparing kaolin clay for use in industry, especially for use in paper coating compositions, and in particular in those procedures which are commonly adopted in the United States of America, the raw kaolin clay is subjected, in aqueous suspension, to various particle size separations. The thus beneficiated fine kaolin product is separated from the water by filtration and the filter cake redispersed in water to form a more concentrated suspension containing from about 50% to about 60% by weight of dry clay. At this stage two alternative methods are available to produce a final suspension which is suitable for transport and storage. In the first method the suspension containing 50-60% by weight of dry clay is spray dried and the spray dried product redispersed in water containing a dispersing agent to provide a suspension containing around 70% by weight of dry clay. In general a final suspension prepared by this method is found to have acceptable rheological properties under conditions of high shear. In the second method, however, the suspension containing 50-60% by weight of dry clay is not spray dried but instead is mixed with sufficient previously spray dried clay to increase the solids content to around 70% by weight. The final suspension prepared by this method generally tends to exhibit a higher viscosity under conditions of high shear than an equivalent suspension prepared by the first method. The present invention provides a way of overcoming the disadvantage of the second method.